The present invention relates to a lithium secondary battery which achieves suppression of deterioration in the charge-discharge cycle characteristic of a battery caused by decomposition of an electrolyte by limiting the moisture mixed in an organic electrolyte solution to a considerably lower level as well as to improvement of its self-discharge characteristic.
In recent years, realization of practical use of lithium secondary batteries is being planned, as secondary batteries with a large energy density, and which can be used as a power source for electric equipment that is small, such as portable communication devices and notebook-sized personal computers. Moreover, concerns about resource saving and energy saving are raised in the background to international protection of the earth""s environment, which is one of the reasons why the lithium secondary battery is expected to serve as a motor drive battery for electric vehicles and hybrid electric vehicles, which are under consideration for active introduction on the market in the automobile industry, etc. Thus, it is eagerly desired to put large capacity lithium secondary batteries, suitable for these uses into early practical use.
In a lithium secondary battery, a lithium transition metal compound oxide or the like is used as a positive active material, while a carbon material such as hard carbon or graphite is used as a negative active material. Upon charging, lithium ions in the positive active material are transferred to and captured by the negative active material through an electrolyte solution obtained by dissolving a lithium electrolyte in a nonaqueous organic solvent. In discharging, the reverse battery reaction occurs.
Here, as an organic electrolyte solution, the carbonic acid ester family such as ethylene carbonate (EC), diethyle carbonate (DEC), or dimethyle carbonate (DMC), is mainly used, while as an electrolyte, lithium fluoride complex compounds, particularly LiBF4, LiPF6, LiAsF6, LiSbF6, etc. are used. It is known that these electrolytes dissolve well into the aforementioned organic solvent, and show relatively high ionic conductivity.
However, the above-mentioned electrolytes are highly hygroscopic, and there are those, like LiPF6, which decompose due to moisture absorption. In addition, these electrolytes are handled carefully in a dry nitrogen atmosphere, etc. since many of them do not dehydrate easily once they have been moisturized, even if it does not result in decomposition.
Even if the electrolytes are strictly controlled, however, when moisture exists in the electrolyte solution, this moisture causes decomposition of electrolytes. For example, in the case where LiPF6 is adopted as an electrolyte, its decomposition separates out HF (hydrogen fluoride) so that HF affects the positive active material to elute a transition metal in the positive active material. Thus, battery capacity decreases due to a chemical change in the positive active material, causing problems such as deterioration of the charge-discharge cycle characteristic.
The control of moisture contained in such an electrolyte solution requires not only quality control by the manufacturer, etc., producing the electrolyte solutions but also strict control at the site where batteries are assembled. Since other battery parts, e.g. the battery case, electrodes, electrode active material, etc., are usually handled under an air atmosphere prior to assembly, the moisture absorbed into these parts will come out and mix with the electrolyte solution.
Furthermore, the present inventors have obtained an experimental result in which mixing of moisture into the inside of a battery affects the self-discharge characteristic badly. FIG. 4, shows the self-discharge characteristic captured according to changes in open circuit voltage in case where the experimental coin cells were formed under various conditions using an electrolyte solution in which LiPF6 was dissolved in a mixed solvent of EC and DEC. Having been left alone after full charging, a battery D, which was formed and charged inside a globe box replaced with and filled with dry nitrogen, shows the least self-discharge, while a battery A, which was formed inside a similar globe box, and was thereafter charged inside a tight box containing a silica gel, proceeds with self-discharge a little bit faster than the battery D.
In comparison with the above, a battery B, which was charged in a tight box which was assembled in an air atmosphere and in which silica gel was put, showed a steep voltage decrease in about half the time of the battery D or the battery A, spends, and in addition, a battery C, which was formed inside said globe box using an electrolyte solution where water drops were intentionally added, and charged within said tight box, showed a steep voltage drop immediately after finishing the charge. It may thus be considered that the moisture within a battery greatly affects the self-discharge characteristic.
Therefore, there is a possibility that the admixture of moisture takes place not only from the materials with which the above-mentioned battery and each member are made, but also from the mixture of moisture inside a battery under the environment where a battery is being produced. Usually, to avoid such an event, the assembly of a battery is performed under a dry nitrogen atmosphere, etc., resulting in, however, considerable cost for production facilities to produce large-capacity large-sized lithium secondary batteries under such an atmosphere.
The present invention was achieved, considering the problems of the prior art mentioned above, the purpose of which is to provide a lithium secondary battery that removes moisture that mixes easily within the battery and that has a good charge-discharge cycle characteristic and a self-discharge characteristic without requiring large-scale production facilities.
That is, according to the present invention, a lithium secondary battery comprising a battery case, an internal electrode body contained in the battery case and including a positive electrode, a negative electrode and a separator film made of porous polymer, the positive electrode and the negative electrode being wound or laminated so that the positive electrode and negative electrode are not brought into direct contact with each other via the separator film, an organic electrolyte solution contained in the battery case, and a zeolite having a moisture absorption characteristic, having been incorporated in the battery case so that the zeolite is brought into contact with the organic electrolyte solution within the battery case.
In a lithium secondary battery of the present invention, it is preferred that the zeolite be incorporated in the battery case, using at least one of the following means, namely: (1) a means to dispose the zeolite to be contained in a bag permeable to electrolyte solution inside the battery case, (2) a means to mix the zeolite with an electrode active material structuring the positive electrode and/or the negative electrode, (3) a means to disperse the zeolite on the surface of the separator film, and (4) a means to make the zeolite into a fine powder and to disperse it by suspension in the electrolyte solution. Thus, it is also preferred to use these means together in plurality.
Here, as the zeolite, the zeolite of an aluminosilicate family having at least a structure of the LTA type, FAU type, CHA type, or MOR type, and having an Al/Si ratio in the zeolite frame equal to or less than 10, is preferably used. Such zeolite does not contribute to battery reaction and exhibits a good moisture absorption under low moisture pressure.